Quote -
If you export the normals the solution is very simple, you only need to create extra vertices in the xported lux files, these extra vertices have the same x,y,z values, the same normal values, but different uv values and the faces at the seam boundary on one side will point to the original vertices and on the other side to the extra vertices. As you export the normals and the normals are the same for both faces for Lux it will be two welded faces even have different vertices number.
The same for different material boundaries..

That's precisely how I'm exporting right now, and it renders fine.

The problem occurs when I tell Lux to subdivide the mesh further using its built-in Loop subdivision algorithm. Because Lux does no automatic welding (as far as I know, but the discussions on the forums seem to confirm that), the subdivision algorithm will treat those vertices as different, even though they have the same positions and normals. So any texture seam or material boundary will look like a mesh boundary (not for normal rendering, mind you, only when subdividing), and as a result one gets creases and sometimes even holes.

Because of that, people are currently forced to do the subdivision in the exporters or directly in the application.

-- I'm not mad at you, just Westphalian.